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Background  ‘ Candidatus  Berkiella cookevillensis’ and ‘ Ca.  Berkiella aquae’ have previously been described as intranuclear bacteria of amoebae. Both bacteria were isolated from amoebae and were described as appearing within the nuclei of  Acanthamoeba polyphaga  and ultimately lysing their host cells within 4 days. Both bacteria are Gammaproteobacteria in the order  Legionellales  with the greatest similarity to  Coxiella burnetii.  Neither bacterium grows axenically in artificial culture media. In this study, we further characterized ‘ Ca.  B. cookevillensis’ by demonstrating association with nuclei of human phagocytic and nonphagocytic cell lines.    Results  Transmission electron microscopy (TEM) and confocal microscopy were used to confirm nuclear co-localization of ‘ Ca.  B. cookevillensis’ in the amoeba host  A. polyphaga  with 100% of cells having bacteria co-localized with host nuclei by 48 h. TEM and confocal microscopy demonstrated that the bacterium was also observed to be closely associated with nuclei of human U937 and THP-1 differentiated macrophage cell lines and nonphagocytic HeLa human epithelial-like cells. Immunofluorescent staining revealed that the bacteria-containing vacuole invaginates the nuclear membranes and appears to cross from the cytoplasm into the nucleus as an intact vacuole.    Conclusion  Results of this study indicate that a novel coccoid bacterium isolated from amoebae can infect human cell lines by associating with the host cell nuclei, either by crossing the nuclear membranes or by deeply invaginating the nuclear membranes. When associated with the nuclei, the bacteria appear to be bound within a vacuole and replicate to high numbers by 48 h. We believe this is the first report of such a process involving bacteria and human cell lines.    Electronic supplementary material  The online version of this article (10.1186/s12866-019-1457-z) contains supplementary material, which is available to authorized users.

Infection and nuclear interaction in mammalian cells by ‘Candidatus Berkiella cookevillensis’, a novel bacterium isolated from amoebae